1. Field of the Invention
This invention is concerned with analysis of complexing agents in electroless plating baths as a means of providing control over the deposit properties.
2. Description of the Related Art
Plating baths are widely used by the electronics industry to deposit a variety of metals (copper, nickel and gold, for example) on various parts, including circuit boards, semiconductor chips, and device packages. Both electroplating baths and electroless plating baths are employed. For electroplating, the part and a counter electrode are brought into contact with the electroplating bath containing ions of an electrodepositable metal, and the metal is electrodeposited by applying a negative potential to the part relative to the counter electrode. For electroless plating, the bath also contains a reducing agent which, in the presence of a catalyst, chemically reduces the metal ions to form a deposit of the metal. Since the deposited metal itself may serve as the catalyst, the electroless deposition, once initiated, proceeds without the need for an externally applied potential.
The electronics industry is transitioning from aluminum to copper as the basic metallization for semiconductor integrated circuits (IC's) in order to increase device switching speed and enhance electromigration resistance. The leading technology for fabricating copper IC chips is the “Damascene” process (see, e.g., P. C. Andricacos, Electrochem. Soc. Interface, Spring 1999, p.32; U.S. Pat. No. 4,789,648 to Chow et al.; U.S. Pat. No. 5,209,817 to Ahmad et al.), which depends on copper electroplating to provide complete filling of the fine features involved.
In the Damascene process, as currently practiced, vias and trenches are etched in the chip's dielectric material, which is typically silicon dioxide, although materials with lower dielectric constants are under development. A barrier layer, e.g., titanium nitride (TiN), tantalum nitride (TaN) or tungsten nitride (WNx), is deposited on the sidewalls and bottoms of the trenches and vias, typically by reactive sputtering, to prevent Cu migration into the dielectric material and degradation of the device performance. Over the barrier layer, a thin copper seed layer is deposited, typically by sputtering, to provide enhanced conductivity and good adhesion. Copper is then electrodeposited into the trenches and vias. Copper deposited on the outer surface, i.e., outside of the trenches and vias, is removed by chemical mechanical polishing (CMP). A capping or cladding layer (e.g., TiN, TaN or WNx) is applied to the exposed copper circuitry to suppress oxidation and migration of the copper. The “Dual Damascene” process involves deposition in both trenches and vias at the same time. In this document, the term “Damascene” also encompasses the “Dual Damascene” process.
Damascene barrier layers based on electrolessly deposited cobalt and nickel are currently under investigation [e.g., Kohn et al., Mater. Sci. Eng. A302, 18 (2001)]. Such metallic materials have higher electrical conductivities compared to metal nitride barrier materials, which enables copper to be electrodeposited directly on the barrier layer without the use of a copper seed layer. Higher barrier layer conductivity also reduces the overall resistance for circuit traces of a given cross-sectional area. In addition, electroless deposition provides highly conformal coatings, even within ultra-fine trenches and vias, so that the overall coating thickness can be minimized. Electroless cobalt and nickel baths being investigated for Damascene barrier deposition typically also contain a refractory metal (e.g., tungsten, molybdenum or rhenium), which co-deposits with the cobalt or nickel and increases the maximum temperature at which effective barrier properties are retained.
For electroless cobalt and nickel baths, hypophosphite (H2PO2−) is typically used as the reducing agent, which introduces phosphorus into the deposit. The codeposited phosphorus reduces the deposit grain size and crystallinity (compared to electrodeposits), which tends to improve the deposit barrier properties. Alternative reducing agents include the boranes, dimethylamineborane (DMAB), for example. Use of a borane reducing agent introduces boron into the deposit. A typical bath for electroless deposition of Damascene barrier layers comprises 0.1 M cobalt chloride or sulfate, 0.2 M sodium hypophosphite, 0.03 M sodium tungstate, 0.5 M sodium citrate, 0.5 M boric acid, and a small amount of a surfactant. Such Co(W, P) baths typically operate at about pH 9 and a temperature of 85°-95° C., and may also contain organic additives.
For electroless deposition of cobalt and nickel on dielectric materials, such as silicon oxide, or on metals that are not sufficiently catalytic for the electroless process, such as copper, a seed layer of a catalytic metal is generally employed. Typically, catalytic palladium is deposited by immersion of the part in an acidic activator solution containing palladium chloride and fluoride ion. The fluoride ion tends to cause dissolution of surface oxides on the substrate so that a displacement layer of palladium is formed. Alternatively, a seed layer of the electrolessly deposited metal, cobalt or nickel, may be applied by sputtering.
Recently, direct deposition of capping layers of Co(W, P) on Damascene copper circuits was reported [T. Itabashi, N. Nakano and H. Akahoshi, Proc. IITC 2002, p. 285-287] for a Co(W, P) bath employing two reducing agents. In this case, electroless deposition is initiated by the more active reducing agent (DMBA), which is present at a relatively low concentration. As the DMBA reducing agent becomes depleted at the part surface, electroless deposition is sustained by the less active reducing agent (hypophosphite), which provides better deposit properties.
Close control of the concentrations of the constituents of electroless plating baths is necessary to provide acceptable deposit properties. Some constituents can be detected by standard analytical techniques whereas specialized methods are needed to measure the concentrations of other constituents. A method for measuring the concentration of reducing agents in electroless plating baths, based on metal electrodeposition rate measurements, is described in a U.S. patent application Ser. No. 10/288,989 to Pavlov et al. (filed Nov. 6, 2002).
Measurement of the concentrations of complexing agents in electroless plating baths is complicated by the presence of reducing agents and buffering agents. Reducing agents tend to interfere with analyses based on redox reactions, whereas buffering agents tend to interfere with acid-base titrations. In addition, complexing agents employed in electroless plating baths are generally weak complexing agents (citrate ion, for example), whose concentrations cannot be readily measured by complexometric titrations. Thus, the conventional approach of determining the concentration of “free” complexing agent by titration with a metal salt solution, and determining the concentration of complexed species by a separate analysis, cannot be used for the relatively weak complexing agents employed in electroless plating baths. Such two-part analyses are undesirable in any case since measurement errors for the separate analyses are multiplied. A method is needed for accurately measuring the concentration of complexing agents, such as citrate ion, in electroless plating baths.
Analysis of various anionic species (sulfate, chromate, molybdate, tungstate, oxalate, phosphate, pyrophosphate and hexacyanoferrate) by addition of fluoride and chloride ions to the unknown solution and titration with lead nitrate is described in the prior art literature [C. E. Efstathiou and P. Hadjiioannou, Analytica Chimica Acta 109, 319 (1979)]. In this case, Pb2+ ions in the titrant react with the anion of interest until it is consumed, and then precipitate PbClF by reaction with the added chloride and fluoride ions. The endpoint for the titration is signaled by a decrease in fluoride ion concentration, which is detected via a fluoride ion specific electrode. In this case, addition of an organic solvent (acetone, ethanol or propan-2-ol, for example) to the unknown solution enhances PbClF precipitation and sharpens the endpoint.
This prior art method has not been applied to analysis of complexing agents in electroless plating baths but would have significant disadvantages for that application. In particular, lead is a toxic metal so that use of Pb2+ ion as a reagent creates safety and environmental issues. Another disadvantage is that the PbClF precipitate is difficult to remove from reaction vessels and can significantly increase the time needed for rinsing between analyses, and/or introduce measurement errors via cross-contamination. In addition, organic solvents used to enhance the precipitation reaction and sharpen the titration endpoint are also objectionable from safety and environmental standpoints.